LIST OF TERMS AND DEFINITIONS

Accelerated erosion.- Erosion at a rate greater than normal for the site, brought about by man, usually through reduction of a vegetation cover.

Accretion. - A process of sediment accumulation by flowing water due to any cause, including alluviation (see lateral and vertical accretion).

Aggradation. - The raising of the surface of stream beds, flood plains, and the bottoms of other water bodies by the accretion or deposition of material eroded and transported from other areas.

It is the opposite of degradation.

Alluvial. - Pertains to alluvium deposited by a stream or flowing water.

Alluvial channel. - See alluvial stream.

Alluvial deposit. - Clay, silt, sand, gravel, or other sediment deposited by the action of running or receding water.

Alluvial fans. - A deposit of loose rock material shaped like a segment of a cone formed be- cause of a sudden flattening of a stream gradient especially at debouchures of tributaries on main stream flood plains. Also known as Alluvial cones.

Alluvial plain. - Plain formed by the deposition of alluvial material.

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Alluvial river. - A river which has formed its channel by the processes of degradation and ag- gradation. The sediment which it carries (except for the wash load) is similar to that in the bed.

Alluvial stream. - A stream whose channel boundary is composed of appreciable quantities of the -- sediments transported by the flow, and which generally changes its bed forms as the rate of

flow changes.

Alluviation.- I_.-- The process of accumulating sediment deposits at places in rivers, flood plains, lakes, or estuaries, where the flow is retarded. See accretion.

Alluvium. - A general term for all detrital deposits resulting from the sediment transport of (modern) streams, thus including the sediments laid down in riverbeds, flood plains, and stream- created fans at the foot of mountain slopes.

Antidunes. - Bed forms which occur at a velocity higher than that which forms dunes and plane beds. Antidunes commonly move upstream, and are accompanied by, and in phase with waves, on the water surface.

Armouring. - The formation of a resistant layer of relatively large particles resulting from removal of finer particles by erosion.

Avulsion. - A sudden, natural change in a stream channel, so that the water flows elsewhere than in its previous course.

Bank. - Rising land bordering a river, lake or sea.

Bar, - Bank of sediment, e.g., sand or gravel, deposited in the stream bed or at its mouth, which obstructs flow or navigation.

Barrage, see also Barrier, Dam, Weir. - (1) Barrier provided with a series of gates or other control mechanisms, across a stream to control the water-surface level upstream, to regulate the flow or to divert water supplies into a canal. (2) See also ground-water dam. A structure which prevents the intrusion of saltwater in a tidal river.

Basin. - Drainage area of a stream or lake.

Beach. - An accumulation of unconsolidated materials found in the zone between land and water, usually recognised as that part which lies between high and low water marks, and formed by the action of the waves.

Bed load. - Material moving on or near the stream bed by rolling, sliding, and sometimes making brief excursions into the flow a few diameters above the bed.

Bed load discharge. - The quantity of bedload passing a transect in a unit of time.

Bed load sampler. - A device for measuring sediment moving on or along the bottom of the river bed.

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Bed material. - The sediment mixture of which the bed is composed. Bed material particles may be moved momentarily or during some future flow condition.

Bed-material load. - That part of the total load of a stream which is composed of particle sizes present in appreciable quantities in the shifting portions of the stream bed. The bed- material discharge would be the bed-material load at the cross section of discharge.

Bed-material sampler. - A device for taking a sample of the sediment of which the stream bed is composed.

Bed-movable. - A stream bed made up of materials readily transportable by the streamflow.

Bend. - Change in the direction of a stream.

Bottom current. - Movement of high density water along the bottom of reservoirs.

Bottomset bed. - Fine-grained material (usually silts and clays) slowly deposited on the bed of a quiescent body of water and may in time be buried by foreset beds and topset beds.

Boulder. - See scale of particle sizes, table 7.5 and figure 7.5.

Braided river. - A wide, shallow and unstable channel where flow passes through a number of small interlaced channels separated by bars or shoals.

Catchment. - The area contributing to ordrainingto a lake, stream, or measuring site (see Basin and Watershed).

Channel. - A natural or artificial waterway which periodically or continuously contains moving water (see Watercourse).

Channel-fill deposits. - Deposits of sediment within a channel, partly or completely filling the channel. Such materials accumulate'where the transporting capacity has been insufficient to remove it as rapidly as it has been delivered.

Channel, regime. - Channel which is in average equilibrium in its cross-section and logitudinal slope. Channel, stable; canal, stable. Channel in which accretion balances scour on the av- erage.

Clay. - See scale of particle sizes, table 7.5 and figure 7..5.

Cobbles. - See scale of particle sizes, table 7.5 and figure 7.5.

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Cohesive sediments.

-___ - Sediments whose resistance to initial movement or erosion is influenced by the existence of cohesive bonds between particles, in addition to the physical properties -of the particles and their relative position with respect to surrounding particles. In the

broad sense, soils bound by a root network and protected by a vegetatsve cover are also inclu- ded.

Colluvial deposits. - Unsorted or poorly sorted deposits accumulated along valley margins by slope wash and by various types of mass movements from the adjacent hillsides.

Concentration of sediment by weight. - The ratio of the weight of dry solids in a water-sedi- ment mixture to the weight of the mixture. This concentration when first determined on a weight basis as p/m (parts per million) can be converted to mg/litre (milligrams per litre) on the basis of table 7.6.

Concentration of sediment by volume. - The ratio of the volume of dry solids in a water-sediment mixture to the volume of the mixture.

Critical tractive force. - 'The minimum force necessary to initiate movement of sediment part- icles in the streambed.

Degradation. - The lowering of the surface of streambeds, flood plains, and the bottoms of other water bodies by the removal of material from the boundary. It is the opposite of aggradation.

Delta - A deposit of sediment, partly above water level formed where moving water is slowed byody of standing water.

Density. - Mass of a substance per unit volume.

Density current. - The movement of fluid under, through, or over another fluid, the density of which differs from that of the moving fluid.

Table 7.5 - Scales of particle sizes

Table 7.6 - Factors for conversion of sediment concentration in parts

Depth-integrating sediment sampler. - An instrument that is moved vertically at an approximat- ely constant rate between the water surface and a point a few inches above the streambed. It collects a representative discharge-weighted water-sediment mixture at all points along the sampling vertical.

Depth integration. - A method of sampling to obtain a representative, discharge-weighted water- sediment sample of stream verticals, except an unmeasured zone near the streambed, by continu-

Diameter, sediment (standard). - See Standard sedimentation diameter.

Discharge-weighted concentration. - The dry weight of sediment in a unit volume of stream dis-

Entrainment. - The process of picking up and carrying along, as the collecting and movement of the sediment (as bedload or in suspension) by currents.

Equal-discharge-increments (EDI). - A procedure for obtaining the discharge-weighted suspended- sediment concentration of flow at a cross section whereby (1) depth integration is performed at the centres of three or more equal flow segments of the cross section, and (2) a vertical tran- sit rate is used at each sampling vertical that will provide equal volumes from all flow seg- ments.

Equal-width-increments (EWI). - A procedure of obtaining the discharge-weighted suspended-sedi- ment concentration of flow at a cross section whereby (1) depth integration is performed at a measured continuously or periodically as an index to discharge and other parameters.

Geological erosion. - The erosion process on or ingiven land form undisturbed by activities of pressed in mass (tonnes) but sometimes expressed volumetrically.

erosion.

Gully - The downcutting process of eroding the soil surface with a deep cut or trench- ed channel below an uncut reach creating a sudden break in the surface slope.

Instantaneous sampler. - A suspended-sediment sampler which essentially instantaneously traps and removes a representative specimen of the water-sediment mixture in a stream at a desired depth and time.

Interfluvial. - The whole catchment area excluding watercourses.

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Isokinetic sampling. - To sample in such a way that the water-sediment mixture moves with no fine-grained, natural levees may be absent or nearly imperceptible.

Nominal diameter. - The diameter of a sphere that has the same volume as the (sediment) parti- cle.

Noncohesive sediments. - Sediments consisting of discrete particles, the movement of which for given erosive forces depends only upon the physical properties of shape, size, and density, and

Point bar. - Loci of sediment deposition on the convex side at river curves (see Lateral accre- tion).

Point-integrating sediment sampler. - An instrument designed to collect a representative sample of the water-sediment mixture at a selected depth in a stream vertical over a specific time period.

Point-integrated sample (point sample). - A water-sediment mixture that is accumulated contin- uously at about the prevailing stream velocity, over a specific time period, in a sampler that surface runoff, ground-water runoff and seepage.

Sampling vertical. - See vertical.

Sediment delivery ratio. - The ratio of sediment yield to gross erosion expressed in percent.

Both sediment yield and gross erosion are expressed in weight units.

Sediment discharge. - The mass or volume of sediment passing a stream cross section in a unit of time. The term may be qualified as suspended-sediment discharge, bedload discharge, or total sediment discharge.

Sediment load. - The solid material sizes that are being moved by a stream through a stream cross section per unit of time. Bed material load plus wash load.

Sediment particle. - Fragments of mineral or organic material in either a singular or aggregate state.

Sediment sample. - A quantity of water-sediment mixture that is collected to represent: (1) the average concentration of suspended sediment, (2) the average size distribution of suspended or deposited sediment, (3) the s pecific weight of deposited sediment, or (4) other sediment para- meters.

Sediment yield. - The total sediment outflow from a watershed or pas-t a given location in a specified period of time. It includes bedload as well as suspended load. Usually expressed in weight per unit of time.

Sedimentation. - A term applied to the five fundamental processes responsible for the forma- tion of sediment, (1) weathering, (2) detachment. (3) transportation, (4) deposition and (5) diagenesis.

Sedimentology. - The scientific study of sediment, sedimentary rocks, and of the processes by which they were formed.

Settling. - The process of deposition of suspended matter carried by water, or other liquids, by gravity. It is usually accomplished by reducing the velocity of the flow below the point where it can transport the suspended material.

Sheet erosion. - The more or less uniform removal of soil from an area by raindrop splash and overland flow without the development of water channels exceeding 30 cm in depth. Included with sheet erosion, however, are the numerous but conspicuous small rills that are caused by minor concentrations of runoff. The rills can be easily obliterated by normal field cultiva- tion. Maximum depth of a rill is 30 cm. Larger water channels are gullies.

Sieve diameter. - The size of sieve opening through which a given particle of sediment will just pass.

Silt. - Individual mineral particles that range in diameter from 0.004 mm to 0.062 mm. See scale of particle sizes, table 7.5 and figure 7.5.

Sloughs. - Areas of dead water, formed in meander-scroll depressions and along the valley walls as flood flows move directly down valley scouring adjacent to the valley walls.

s. -

Unconsolidated mineral and organic material underlying the surface that has been suf- ficiently modified and acted upon by physical, chemical, and biological agents that it will support plant growth.

Sorting. - The dynamic process by which sedimentary particles having some particular character- istics (such as similarity of size, shape or specific gravity) are naturally selected and sep- arated from associated but dissimilar particles by the action of flowing water.

Sphericity. - (Shape Factor) True sphericity is the ratio of the surface area of a sphere with the same volume as a grain to the surface area of the grain. A more convenient expression is the ratio of the diameter of a circle with an area equal to that of the projection of a grain when it rests on its larger face to the diameter of the smallest circle circumscribing this pro- jection.

Splay. - Deposits of flood debris usually of coarser sand particles in the form of scattered deposits on the flood plain.

Standard fall diameter. - The diameter of a sphere that has a specific gravity of 2.65 and has the same standard fall velocity as the particle.

Standard fall velocity. - The average rate of fall that a particle would finally attain if fal- ling alone in quiescent distilled water of infinite extent and at a temperature of 24-C.

Standard sedimentation diameter. - The diameter of a sphere that has the same specific gravity and has the same fall velocity as the given particle.

Streambank erosion. - The removal of stream channel bank material caused by the force of flowing water and the caving of stream banks.

Stream discharge. - The quantity of flow passing through a cross section of a stream in a unit of time. (The natural water contains both dissolved solids and sediment).

Suspended-sediment concentration by weight. - See concentration of sediment.

Suspended-sediment discharge. - The quantity of suspended-sediment passing through a stream cross section in a unit of time.

Suspended-sediment load. - The weight of suspended particles which is continuously supported by the water.

Suspended-sediment sample. - A quantity of water-sediment mixture representative of the concen- tration and particle-size distribution of suspended sediment in the water sampled.

Suspended-sediment sampler. - A device that collects a representative sample of the water with its suspended-sediment load.

Texture. - The geometrical aspects of the component particles of a sediment deposit or rock including size, shape, and arrangement.

Terminal velocity. - The limiting velocity reached asympototically by a particle falling under the action of gravity in a still liquid at a specified temperature.

Thalweg. - The line connecting the lowest or deepest point along a stream bed or valley, whether underwater or not.

Topset bed. - A layer of sediment deposited on the top surface of an advancing delta which is continuous with the landward alluvial plain.

Total sediment discharge. - The total quantity of sediment passing a section in a unit of time.

Total sediment load (total load). - The total sediment in transport in a stream; that part mov- ing as suspended load plus that moving as bed load.

Traction. - A general term for that mode of transport of debris by running water, in which the particles are swept along close to the bed of the stream by rollrng, sliding, or saltation (see 'ded load).

Tractive force. - Force exerted by flowing water on the sediment particle at standstill on the river bed to induce movement.

Transect. - A cross section of an area used as a sample to indicate one or more specific char- acteristics of the area.

Trap efficiency. - The proportion of the incoming sediment load deposited in a lake or reserve oir, in percent.

Transportation -- - The complex process of moving sediment particles from place to place by moving water. The principal factors affecting transportation are turbulence, ratio of settling velocity to water velocity, shape, size, density, and quantity of particles and saltation.

Turbidity. - An expression of the optical properties of a sample which causes light rays to be scattered and absorbed rather than transmitted in straight lines through the sample. Turbidity of water is caused by the presence of suspended and dissolved matter such as clay, silt, finely divided organic matter, plankton, other microscopic organisms, organic acids, and dyes.

Turbidity current. - Density current caused by high turbidity.

Turbulence. - The irregular motion of a flowing fluid.

Unit weight (sediment deposits). - Weight of sediment in terms of the dry, wet, or saturated mass of solids per unit volume of deposit in place.

Unmeasured sediment discharge. - The difference between the total sediment discharge and the measured suspended sediment discharge.

Unsampled zone. - The unsampled part of the sampling vertical; usually, assumed to be 9 to 15 cm above the stream bed depending on the kind of sampler used.

Valley trenching. - Gully erosion occurring in flood plains.

Vertical. - An approximately vertical path from water surface to stream bed along which one or more samples are taken to define sediment concentration or distribution.

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Vertical accretion deposits. - Flood plain deposits formed by deposition of suspended sediment from overbank flood waters.

Volume weight. - Specific weight, apparent density, dry-bulk density are synonymous.

Wash load. - The portion of the sediment load of a stream composed of the finer particles (usually finer than 0.062 mm) which are found only in relatively small quantities in the bed.

Practically all of the wash load is carried in near-permanent suspension and its magnitude depends primarily on the amount of fine material available to the stream from the sources other than the bed.

Water course. - A natural or artificial waterway varying in size from a relatively shallow channel to large river which intermittently or continuously contains moving water.

Water discharge. - The quantity of water passing a cross section in a unit of time. (The nat- ive water contains both dissolved solids and sediment). (See Stream discharge).

Water pollution. - The addition of harmful or objectionable material to water in sufficient quantities to adversely affect its usefulness.

Watershed. - All lands enclosed by a continuous hydrologic surface drainage divide and lying upslope from a specified point on a stream (sameas Drainage basin).

Weathering. - A process of breaking up of the weaker or softer rocks by atmospheric activities and gravity.

Well graded. - A continuous distribution of grain sizes from the coarsest to the iinest compon- ents in such proportions that the successively smaller grains just fill the spaces between the larger grains (Same as Poorly sorted).

REFERENCES

Battelle Colombus Laboratories. 1970. Review of the state-of-the-art in nonpoint source contr- ols in the U.S., 1978.

Lane, E.W. et al. 1947. Report of the Subcommittee on Sediment Terminology, Trans. Am. Geophys.

Un. vol. 28 no. 6.

FAO. 1977. Conservation Guide 1. Guidelines for Watershed Management. Rome, Food and Agricul- ture Organisation of the United Nations.

Toy, T.J. et al. 1977. Erosion: Research Techniques, Erodibility and Sediment Delivery. Geo.

Abstracts Limited, Norwich, England.

U.S. Department of Agriculture. Present and Prospective Technology for Predicting Sediment Yields and Sources. Agr.Res.Serc. Publ. ARS-S-40. June 1975.

U.S. Environmental Protection Agency. Loading Functions for Assessment of Water Pollution from Nonpoint Sources. October 1973.

Wischmeier, W.H.;Smith, D.D. 1978. Predicting rainfall erosion losses. SEA, U.S. Dept. of Agriculture. (Agriculture Handbook no. 537).

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